A photovoltaic device (including a solar energy battery) is different from the regular battery. A photovoltaic device converts light into electrical energy, its P-N junction structure when exposed to incident light generates large quantities of electron-hole pairs, and in the meantime electrons carrying negative electricity and holes carrying positive electricity migrate to the N-type semi-conductor and P-type semi-conductor respectively. This process produces electricity.
During the conversion process, not all the incident sun-light can be absorbed and converted to electricity by the photovoltaic device. One section of the light spectrum does not contribute to the battery's output due to its low energy (which should be greater than semi-conductor's energy gap). When the photovoltaic device receives light of that particular section of the spectrum, instead of converting its energy into electron-hole generation, the photovoltaic device can only release its energy in the form of heat. In the laboratory, the photovoltaic device can almost reach the highest light-electricity-conversion level theoretically allowed, but in reality due to the complexity of the mass manufacturing process cost-effective high-efficient photovoltaic devices cannot be produced. Currently, this is the biggest bottleneck in making photovoltaic devices.
At present, the development of photovoltaic devices can be divided into two areas; one area focuses on how to capture light and convert the light energy to electricity efficiently; the other area focuses on how to reduce cost in material and manufacturing processes. The former area can be further divided into the following sub-categories: reducing incident light reflection; improving the design of the metal electrode or adding an anti-reflection layer between component layers to reduce reflection and increase the amount of light getting into the semi-conductor material; or reducing inner resistance of the photovoltaic device to increase conversion efficiency, wherein metal electrodes are imbedded in the substrate to increase the contact surface while reducing serial resistance. Two or more photovoltaic devices can be combined to form a tandem cell, where the device absorbing the high-energy spectrum is placed on top while the device absorbing the low-energy spectrum is placed on the bottom; together, they form a high voltage photovoltaic device.